1. Field of the Invention
The present invention relates to a varactor element having a junction region, in which the depletion capacitance of the varactor element varies when a reverse bias voltage is applied to the varactor element. Such a varactor element is also known as varactor diode, tunable diode, or voltage controlled capacitor.
1. Prior Art
Such varactor elements are known, and the behavior is well understood. In textbooks, it can be found that for diode based varactor elements, the capacitance voltage characteristic is in the form of
            C      ⁡              (        V        )              =          K                        (                      ϕ            +            V                    )                m              ,in which C(V) is the capacitance as function of the total (reverse) voltage V across the diode, φ is the built-in potential of the diode, m is the power law exponent of the diode capacitance and K is the capacitance constant. For a diode with a uniform doping profile, m=0.5, and for a diode with a hyper-abrupt junction, m≈1.5. Such a characteristic however limits the application of varactors in certain high quality applications, such as low distortion varactor stacks, especially when used in devices designed for operation with narrow tone spacing signals, such as in many modern day communications systems (see e.g. K. Buisman, L. C. N. de Vreede, L. E. Larson, M. Spirito, A. Akhnoukh, T. L. M. Scholtes and L. K. Nanver, “‘Distortion free’ varactor diode topologies for RF adaptivity,” in 2005 IEEE MTT-S Int. Microwave Symp. Dig., Long Beach, Calif., June. 2005).
In order to overcome these drawbacks, recently varactor diode-based circuit topologies and a high performance varactor diode process technology has been presented, which, for a given diode power-law capacitance coefficient (m≧0.5), can act as variable capacitors with extremely low distortion. However, the proposed solutions have linearity constrains for modulated signals or signals with narrow tone spacing when considering practical implementations. The invention presented here aims to overcome these limitations, in terms of linearity for narrowband signals, sensitivity to leakage currents of the varactors, high control voltage and capacitance tuning range.